Monitoring early-age acoustic emission of cement paste and fly ash paste

Highlights

• Connection between the early age processes in cement paste and the acoustic emission signals.

• Comparison between cement paste and fly ash suspension to separately study the chemical reaction and the shrinkage.

• Parameters of the early-age acoustic emission like frequency and rise time are sensitive to the different processes.

• Acoustic activity of cement paste for the first hours is mainly attributed to the settlement.

Abstract

In this study, a combined approach of several monitoring techniques was applied to allow correlations between the AE activity and related processes such as shrinkage and settlement evolution, capillary pressure and temperature development in fresh cementitious media. AE parameters related to frequency, energy, and cumulative activity which exhibit sensitivity to the particle size distribution of cement paste are compared with inert fly ash (FA) leading to isolation of the mechanical sources from the chemical ones. Characterization of the origin of different processes occurring in cement paste during hydration is complex. Although acoustic emission (AE) monitoring has been used before, a qualitative relation between the microstructural formation or other early-age processes and the number or parameters of AE signals has not been established. The high sensitivity of AE enables the recording of elastic waves within the cementitious material, allowing the detection of even low-intensity activities.

Introduction

The early age hydration and the associated microstructural formation of concrete are the dominant factors for prediction of concrete durability and service life. Isolating the different mechanisms such as hydrates production, microstructure evolution and other thermomechanical changes taking place during setting and hardening, appears to be a challenging task. The risk of early age cracking arising from the development of stress concentrations is high at the initial stages of hydration [1]. Therefore, understanding and isolating the different mechanisms of early hydration is significant as the long-term behaviour of concrete is closely related to its early age properties. During this period, an exothermic reaction between cement particles and water takes place leading to the formation of hydration products with a complex microstructure [2]. Isothermal calorimetry [3] and thermal analysis [4] can provide relevant information on the different chemical processes occurring during hydration. In addition, a sensitive and precise technique is essential to characterise the influence of early-age processes like settlement and hydration which are associated with microstructural transformation and the mechanical properties and durability at a later stage. Different techniques have been applied to monitor the early hydration, such as linear and nonlinear ultrasonics [[5], [6], [7], [8]], scanning electron microscope [9], X-ray computed tomography [10], electrical impedance measurement [8], etc. Lu et al. [11] investigated the hydration process of early age concrete using an embedded active acoustic method along with non-contact complex resistivity and identified four distinctive hydration stages.

In recent years, researchers have also applied the AE technique to monitor the hydration process. AE collects low energy elastic waves from irreversible changes in a material and is very promising due to its sensitivity to the micro-scale processes. Chotard et al. [12] applied AE to characterise the early hydration and the related internal changes occurring in a cement paste (0-24 h). AE signals were associated with the emptying of the capillary pores, the formation and growth of the hydrates and the development of microcracking. Moreover, AE has been used to predict the mechanical behaviour of a set cement from its early age properties since the number of recorded signals was attributed to the amount of hydration products [13,14]. Libor et al. [15] studied acoustic emission activity during hardening of cement-based composites and observed that the higher the AE signal amplitude values, or the AE signal energy, the more significant structural changes arise in the form of newly formed hydrates. AE has shown sensitivity to the w/c weight ratio. Cement pastes with lower w/c weight ratio demonstrated higher AE activity in comparison to the other samples and this observation was attributed to cavitation in the cement paste pores [16]. Concrete with higher aggregate to cement ratio demonstrated lower cumulative AE hits [17]. Qin et al. [18] monitored the cracking activities of early age concrete and defined three hydration stages according to the accumulated recorded AE signals. AE parameters of “signal strength”, amplitude and duration revealed a correlation with the temperature gradients during cement hydration [19]. The temperature evolution showed a correlation to the evolution of cumulative AE events for cement paste, cement mortar, and concrete. It was also found that AE activity decreases with a decrease in the heterogeneity of the material [20]. At the time of early age cracking AE exhibited higher amplitude with longer duration signals in restrained cement mortars [21]. Higher numbers of microcracks during the hardening and setting of concrete caused higher numbers of AE events [22]. Two mechanisms of mechanical origin have also been targeted: the movement of aggregates and the movement of bubbles through fresh cement paste were isolated, and their characteristics were studied [23] showing that AE activity from both processes can be well received, especially if they occur close to the metal mould that acts as a waveguide. Furthermore, AE has been used for characterizing the deformation of granular media due to jamming of grains and frictional slip of particles which release strain energy [24]. Different features in the evolution of the AE parameters can be connected to the changes in the hydrates production caused by chemical reactions, including the progressive mechanical setting of the specimen.

This paper aims to investigate the connection between the early age processes in cement paste and the AE signals. This is attempted by comparing measurements made on cement paste specimens with a purely FA suspension to separately study the chemical reaction (in cement paste) and the physical phenomena like settlement and shrinkage occurring to FA suspension as well. In addition, the pulse velocity for all types of materials was monitored at the early stage. The processes occurring in freshly cast cement paste as well as during the curing period was studied and significant correlations to AE parameters, such as average frequency, rise time, absolute energy, and cumulative hits were found. These parameters are explained in the next paragraph.